Dynamic optical interconnect system reference signal formation

ABSTRACT

The disclosed technology provides a dynamic interconnection system which allows to couple a pair of optical beams carrying modulation information. In accordance with the disclosed technology, two optical beams emanate from transceivers at two different locations. Each beam may not see the other beam point of origin (non-line-of-sight link), but both beams can see a third platform that contains the system of the disclosed technology. Each beam incident on the interconnection system is directed into the reverse direction of the other, so that each transceiver will detect the beam which emanated from the other transceiver. The system dynamically compensates for propagation distortions preferably using closed-loop optical devices, while preserving the information encoded on each beam.

This application is a continuation in part of U.S. patent applicationSer. No. 09/848,563 filed May 3, 2001 now U.S. Pat. No. 7,113,707, thedisclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates, in general, to the field of opticaltelecommunications and compensated imaging. It relates to a system andmethod for creating an optical link between two stations, each stationnot necessarily being in the line of sight of the other, with fullduplex communications being possible, and more particularly relates tothe transmission of reference information with associated data.

BACKGROUND

The prior art includes systems for relaying optical information betweentwo beacons. This is conventionally accomplished by first detecting anddemodulating the optical information received by the first beacon froman optical source, subsequently synthesizing a optical beam bymodulating another optical source with this information, and, finally,directing the new optical beam to the second beacon. This multi-elementrepeater system has application to well-defined relay modules, alongoptical fiber links for example, or for N×M interconnects for photonicnetworks, among others. However, in the general case, where propagationerrors may be dynamic, and where the incident beams can arrive over alarge field-of-view, a more robust interconnection system is required.These problems and limitations are addressed by this invention.

The prior art also includes systems comprising a set of tilt-mirrorcompensators which are used for correcting certain errors. Such systemscan only compensate for the lowest-order errors such as tilt or beamwander. Other low-order errors, such as focus and astigmatism, can becorrected with a variable focus element. However, these systems areunable to compensate for higher order propagation errors such as generalwavefront distortions due to propagation through turbulent atmospheres,multi-mode optical fibers, etc. Thus, a system and method are neededthat provide ways of compensating for these errors.

The disclosed technology addresses the general case of phase (wavefront)errors. In this connection, the prior art includes the Double-pumpedPhase Conjugate Mirror (DPCM). The DPCM does not require any servo-loopdevices, since it proceeds via an all-optical nonlinear interaction.However, the DPCM requires the power carried by the incident laser beamto be above a given threshold, in order to properly function. Thisthreshold generally ranges between a few μW/cm² to a few mW/cm²,depending on the particular crystal used for the DPCM. Some examples ofadequate crystals include BaTiO₃ and InP. Moreover, the response time ofa DPCM depends on the intensity of the incident beams, and theintensities of the two incident beams need to have similar values forthe device to function optimally (fast response time, stable operation,and suitable wavefront compensation). Finally, the DPCM is lossy and theinsertion loss can be large, approaching 30% or more.

In contrast, the present device may have a very low insertion loss (itpreferably only requires enough light to be sensed by the wavefronterror sensor which can approach the shot-noise limit per pixel), and canfunction with input beams with intensities which need not to be equal(i.e., not necessarily balanced). Similar to conventional adaptiveoptical systems, the wavefront compensation capability will be afunction of the number of equivalent pixels, or phase actuators,relative to the number of resolvable coherent phase patches which needto be phased up or corrected.

One object of the present invention is to provide a system and methodfor relaying optical information from one transceiver to another.Specifically, this invention will direct a first optical beam emanatingfrom a first transceiver and travelling to a second transceiver, intothe reverse direction of a second optical beam emanating from the secondtransceiver and traveling to the first transceiver. The beams can beencoded, so that a communication link is realized withdiffraction-limited capability. In its most basic form, a simple pair oftilt mirrors may be used to direct one beam into the reverse directionof the other. However, in general, the beams are not plane waves, andmay have undergone time-varying (i.e., dynamically varying) propagationdistortions, including atmospheric distortions, multi-mode fiber-induceddistortion, etc. Therefore, an adaptive optical element is used tocompensate for, and to track out, these undesirable time-varyingdistortions, and, at the same time, provide a means for coupling thelight from one direction into the other, and vice versa (without loss ofthe desired duplex modulation). Since this system provides for couplingof the two optical beams, no local detector or source is required at thelocation of the interconnect module. Reference data is preferablytransmitted with the desired data to be transmitted for tracking out theaforementioned errors. The optical beams that leave the interconnectmodule may propagate in precisely the reverse direction of the incidentbeams (i.e., they are mutually phase-conjugate replicas of the incidentbeams). Thus, pointing and tracking is realized with this system, sothat the system performance is not compromised (i.e., low insertion lossand high directivity). Finally, modulation is preserved on the variousbeams, so that information can be transferred from one station toanother station, with diffraction-limited performance, and subject totypical adaptive optical design issues and constraints, such asdiffraction, dispersion, depolarization, the compensation bandwidth, thespatial bandwidth of the system (e.g., the number of resolvable pixelsfor wavefront reconstruction), etc.

Applications of the disclosed technology include optical “relay nodes”for free-space, space-based or terrestrial-based, as well as forguided-wave based (e.g., coupling of the output of a single ormulti-mode fiber to another fiber or to a free-space path), opticalcommunication and image relay links, or combinations thereof. Manyapplications do not afford the luxury of line-of-sight viewing betweenthe stations at the end points of the communication link. For example, amountain may obstruct the end points for direct viewing, or twosatellites may no longer “see” each other. To overcome this problem, anintermediate “relay node” or interconnection system is required, whichmay be placed on a hilltop or on an intermediate satellite, such thatthe interconnection system is in the line of sight of both stations. Itmay also be necessary to optically relay (one-way or two-way)information from one subsystem (e.g., a multimode fiber) into anothersubsystem (e.g., an array of optical modulators, detectors, etc.).General extensions of this design philosophy follow. For example, acascade of interconnection modules can be placed on a series of hilltopsso that a complete communications link can be established across thechain of hilltops.

As shown in FIG. 1, the prior art discloses a method to first detect anddemodulate the beam (originating from a first station) at theapproximate mid-point (e.g., a hill-top or satellite in the case of anon-fibre based communication system) of the link between two stations,then to encode this information onto another laser, and finally directthe encoded data to a second station to complete the link (on the otherside of the hill-top). This approach, however, does not compensate forpropagation distortions. Hence, the very photons from one end of thelink will arrive at the other end of the link in a diffraction limitedmanner, and, vice versa.

The disclosed technology provides for an automatic re-directing of thebeam, as it arrives at the hill-top, to the second half of the link, asshown in FIG. 2. Moreover, the invention compensates for propagationdistortions, so that the beam will arrive at the other end of the linkwithout distortion. The disclosed technology enables such anintermediate node to be realized, without the usual photonic repeaterrequirements of high-bandwidth photo-detection, modulation andretransmission of the data. In the disclosed technology, the temporalmodulation format imposed onto the beam from its initial point of originis preserved. As it goes through the interconnection system only itswavefronts are modified, while its temporal encoding is maintained.Further, the system can function using mutually incoherent sources(e.g., free-running lasers at each end point of the link). When both ofthese lasers impinge onto the system, the beam from one of theend-points will be directed into the wavefront-reversed direction of thepath that the second beam took, thereby “finding” and arriving to theother end of the link distortion-free (assuming usual time scale of beamformation by the system, range, atmosphere distortion time scale, andmotion of the source locations during the optical transit time). Hence,the very photons from one end of the link will arrive at the other endof the link in a diffraction-limited manner, and, visa-versa.

Additionally, the system of the disclosed technology provides for“auto-tracking”. Indeed, if the end-point stations are moving, theinterconnect can track or follow the moving stations. This assumes thatthe stations move slowly with respect to the reconfiguration time of theinterconnect and the time/spatial scale of the dynamic distortions. Thesystem provides for propagation-distortion compensation as well.

A related application is in the area of space-based low-cost relaymirrors. A pair of large-area telescopes are used to collect a weaksignal, and then relay the beam to another location. These lightweightmirrors, which may be made of thin membranes (mylar, etc.), oftenpossess optical distortions because the lightweight material they aremade of can easily deform. The system performance is thereby degraded.By placing the proposed invention between the pair of large-area relaymirrors, the local mirror aberrations, as well as path distortionsexperienced by the two incident beams, can be corrected in real-time.Other potential areas of application include stratospheric relayplatforms, such as Low Earth Orbit (LEO) and Medium Earth Orbit (MEO)satellites and other airborne systems, with application to backbonefeeder lines, as well as dynamic links for optical fibers, laser sourcesand beam combining systems. In the latter case, a given incident probebeam can be used as a local reference beam, which can, as a result ofthe interconnection system, phase up a collection of single-frequency,but randomly phased oscillators, including optical fiber amplifiers andoscillators.

SUMMARY

The disclosed technology provides a novel system that can adaptivelyinterconnect two incoherent optical beams thereby creating an opticallink between two stations. The disclosed technology also provides amethod of optically interconnecting two stations from which two opticalbeams emanate, the two optical beams being directed from the twostations to a common location such as a hilltop.

The overall scope of the disclosed technology is to provide a dynamicinterconnect capability to couple a pair of spectrally narrowband orbroadband optical beams, or one in each category, which may carrymodulation information. By way of an example, let us suppose that twooptical beams emanate from transceivers at two different locations, andare both incident upon the optical interconnection system of thedisclosed technology. The system will direct each beam into the reverse(i.e., phase conjugate) direction of the other, so that each transceiverwill detect the beam that emanated from the other station without anintermediate detection, demodulation and encoding of another mid-pointsource with the demodulated information.

In general, the incident beams propagate along different paths, and,thus, may experience different propagation distortions, beam wander,etc. The disclosed technology provides an interconnection system foroptical beams which may have experienced different Doppler shifts,possess different wavefront distortions, speckle, as well asdepolarization (the latter two cases would involve the use of additionalSLMs (Spatial Light Modulators).

The system architecture comprises a pair of closed-loop Adaptive Optical(AO) modules (or, two regions on a common-focus correction module, thelatter for bore-sighting the two beams and adaptive optical element), inconjunction with an optional tilt-focus compensator for low-orderaberration errors, if necessary. Also comprised in the system are anumber of reflectors and beam splitters. Each AO module is controlled bya given input beam.

The disclosed optical system is not a conventional repeater device. Thatis, it does not merely detect and demodulate the beam, and then encodethe information onto another optical source (e.g., as in a relaystation). Instead, it re-directs one optical beam into the reversedirection of another by modifying its wavefronts. In this manner, thesystem compensates for wavefront errors along the paths of the twoincident beams, resulting in a well-defined output beam, withnear-diffraction-limited performance. Moreover, any global modulationinformation is preserved on each incident beam, which is redirected intothe reverse path of the other beam. Therefore, no demodulation andsubsequent repeater-based modulator elements are required, therebygreatly simplifying the basic system architecture. If necessary,however, optical amplifiers (bulk or guided-wave classes) can be placedat any point along the system (including at the interconnect module).The interconnect module will provide compensation for opticaldistortions in the amplifiers as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art system for exchanging optical informationbetween two stations, the line of sight between the two stations beingobstructed by a hill.

FIG. 2 demonstrates that an optical interconnect may be used forproviding optical information exchange between two stations, even thoughthe line of sight between the two stations is obstructed by a hill.

FIG. 3A depicts an embodiment wherein the AO modules function inreflection mode and wherein a pair of optical tilt-focus errorcompensators is used;

FIG. 3B depicts an embodiment wherein the AO modules function inreflection mode and wherein a single optical tilt-focus errorcompensator is used;

FIG. 4 depicts an embodiment wherein the AO modules function intransmission mode.

FIG. 5 depicts an embodiment wherein a single AO module is used, the AOmodule comprising a single AO wavefront corrector having two regions,and a pair of wavefront error sensors.

FIG. 6 a depicts one embodiment of the present invention by which areference signal can be transmitted along with desired data to betransmitted, the two signals being distinguished by temporal shifting orsequencing.

FIG. 6 b depicts a preferred format for a frame of transmitted data inthe embodiment of FIG. 6 a.

FIG. 7 shows a alternative embodiment of the present invention by whicha reference signal can be transmitted along with desired data to betransmitted, the two signals being distinguished by polarization of theoptical signals.

FIG. 8 depicts an embodiment of the present invention by which thereference signal can be transmitted along with desired data to betransmitted, the two signals being distinguished by frequency shiftingthe optical signals.

DETAILED DESCRIPTION

Basic embodiments of the disclosed technology is illustrated withreference to FIG. 3A, FIG. 3B, and FIG. 4. The systems of FIGS. 3A and3B relate to an optical interconnect functioning in reflection mode,whereas the system of FIG. 4 relates to an optical interconnectfunctioning in transmission mode. The following description appliesequally to both the reflection-mode systems of FIGS. 3A and 3B and thetransmission-mode system of FIG. 4. When appropriate the distinctionsbetween these two systems are made clear. FIG. 5 depicts an embodimentwherein a single AO module is used, the AO module comprising a single AOwavefront corrector having two regions, and a pair of wavefront errorsensors.

Improvements are discussed with reference to FIGS. 6-8 whereby areference signal can be transmitted along with the desired data to betransmitted.

For the purpose of illustration, beam 9 originating from station A, andbeam 10 originating from station B, are shown displaced relative to oneanother. In actuality, the two beams travel on top of one another, inopposite directions.

The system allows two stations, A and B, to exchange information via anoptical link created between them using an interconnect. Theinterconnect preferably comprises a pair of Adaptive Optical (AO)modules 3 and 4, each of which comprising a pair of AO wavefrontcorrectors 3 a and 4 a, and a pair of Wavefront Error Sensors 3 b and 4b to drive AO wavefront correctors 3 a and 4 a, respectively. Theinterconnect further comprises a pair of optical tilt-focus errorcompensators 7 and 8, placed upstream and downstream of the AO modules,respectively, and a pair of beam splitters 17 and 18 placed between theAO modules. Tilt-focus error compensator 7 is positioned between stationA and AO module 3 such that tilt-focus error compensator 7 is in thelight path between station A and AO module 3. Similarly, tilt-focuserror compensator 8 is positioned between station B and AO module 4 suchthat tilt-focus error compensator 8 is in the light path between stationB and AO module 4. Alternatively, the configuration shown in FIG. 3B maybe used wherein a single optical tilt-focus error compensator 78 is usedinstead of two. In this case, the optical tilt-focus error compensator78 is placed near the midpoint of the overall system, in the light pathbetween AO module 3 and AO module 4, approximately midway between thetwo modules. Beam 15, resulting from the reflection of beam 9 by AOwavefront corrector 3 a, is split by beam splitter 17 into a first partwhich is directed to AO module 4, and a second part which is directed toWavefront Error Sensor (WES) 3 b. WES 3 b senses the distortions (e.g.,its wavefront errors) of the beam, computes the required correction andaddresses the AO wavefront corrector 3 a to drive the input distortionsto zero or nearly zero, depending on the servo-loop gain of the system.Similarly, beam 16, resulting from the reflection of beam 10 by AOwavefront corrector 4 a, is split by beam splitter 18 into a first partwhich is directed to AO module 3, and a second part which is directed toWES 4 b. WES 4 b senses the distortions of the beam, computes therequired correction and addresses the AO wavefront corrector 4 a todrive these input distortions to zero or near zero. This is an exampleof a servo-loop or closed-loop system.

Each AO module 3,4 is driven or controlled (i.e., configured, in termsof its pixelated phase map) by the respective incident optical beam. AOmodule 3 is controlled by optical beam 9 originating from station A,while AO module 4 is controlled by optical beam 10 originating fromstation B.

Preferably, the AO modules 3,4 are configured by a reference signalwhich is subjected to the same path distortions as is the data. Theassumption which is made is that if the distortions in the referencesignals can be adequately compensated for, that applying the samecompensation to the data signals will also compensate for pathdistortions there as well. Different techniques for transmitting thereference information will be discussed later.

In order to optimize the efficiency of the system, beam splitters 17 and18 are preferably designed to transmit most of the incident light(typically in the range of 90% of the incident light, depending on thesignal-to-noise ratio (SNR) achieved) to AO module 4 and AO module 3,respectively, while reflecting just enough light to WES 3 b and WES 4 b,respectively, so that the WESs can function with adequate SNR (i.e.,SNR>1, preferably in the range of 10 to 100 or more).

The purpose of the optical tilt-focus error compensators 7 and 8, asshown in FIG. 3A, is to remove overall tilt and/or focus errors betweenthe pair of beams, so that they propagate in exact opposition to eachother within the system (i.e., they counter-propagate). Thesecompensators 7 and 8 may be omitted if the field-of-view and the dynamicrange of the AO modules 3 and 4 provide sufficient correction forlower-order errors (tilt and focus) without compromising the ability tocompensate for higher-order wavefront errors on the respective inputbeams. This assumes that the AO modules have sufficient dynamic range(i.e, greater than a wave, preferably greater than several waves).

For the purpose of illustration, let us suppose that incident inputbeams 9 and 10, each possesses an arbitrary wavefront error uponincidence onto the respective AO modules 3 and 4. Moreover, let usassume that each beam is encoded with information, in the form of eitheramplitude or phase modulation. The information can be encoded onto asingle spatial mode, or, can be in the form a multi-pixel “image” witheach resolvable pixel corresponding to an independent channel ofinformation.

Assume further that the temporal encoded modulation bandwidth exceedsthe adaptive optical closed-loop compensation bandwidth, so that thedesired modulation is preserved, after beam error compensation. Thecompensation bandwidth must equal or exceed the distortion effectivebandwidth for the system to function. As an example, atmosphericdistortions have a time scale on the order of a millisecond, so the AOcompensation bandwidth must be greater than 1 KHz. On the other hand,the desired communication bandwidth (or link data rate) can be verylarge (1 to 100 GHz, for example).

The role of the AO module 3 is to minimize, upon reflection/transmissionby/through AO module 3, the wavefront errors carried by the input beam9. For example, AO module 3 will drive the spatial phase error φ_(res)of incident beam 9 to a small residual value dictated by the closed-loopservo gain G (φ_(res)≈φ_(in)/(1+G), where φ_(in) is the input phaseerror). The gain G usually ranges from about 2 to about 100, with highervalues of G giving better system performance. The result of thisoperation is that a highly aberrated input beam 9, will, afterreflection by/transmission through AO module 3, emerge as a near-planewave beam 15. Note that any global phase or intensity modulation willremain on the planarized (i.e., the wavefront scrubbed) beam 15. Theplanarized beam 15 maintains the (desired) globally encoded modulationinformation. This modulated plane wave beam 15 will then bereflected/transmitted by/through the other AO module, namely AO module4. Note that the cleaned-up beam 15 does not affect the spatial phase ofAO module 4 since this module is controlled by incident beam 10originating from station B.

By reciprocity, the plane wave beam 15 will, upon reflectionoff/transmission through AO module 4, emerge with the same wavefront asbeam 10 had before it reflected off/transmitted through AO 4. Theencoded input beam 9 will thus propagate into the precise reversedirection of beam 10 and arrive at station B as a diffraction-limitedbeam. Diffraction-limited characterizes a beam with highest focusingability, and is determined by the ratio, λ/D, where λ is the wavelengthand D the aperture. Optical distortions increase this ratio by one toseveral orders of magnitude (˜10 to ˜1000, or more) which in turndegrades performance.

The foregoing discussion is also applicable to AO module 4, input beam10, planarized beam 16 and station A.

In yet another embodiment of the disclosed technology, the two AOwavefront correctors 3 a and 4 a of FIGS. 3A, 3B and 4, are replacedwith two regions on a common-focus correction module, as illustrated inFIG. 5. In accordance with this embodiment, the interconnect comprises acommon-focus correction module or AO wavefront corrector 34 having afirst region 341 and a second region 342, each region forming a separateAO wavefront corrector. The interconnect further comprises a pair ofWESs 34A and 34B, to drive AO wavefront corrector regions 341 and 342respectively, a pair of optical tilt-focus error compensators 7 and 8,placed upstream of the AO wavefront corrector 34, a pair of beamsplitters 38 and 39, and seven reflectors 35, 36, 37, 40, 41, 42 and 43.Tilt-focus error compensator 7 is positioned between station A and AOwavefront corrector 34 such that station A, tilt-focus error compensator7, and region 341 of the AO wavefront corrector 34, are substantiallyaligned. Similarly, tilt-focus error compensator 8 is positioned betweenstation B and AO wavefront corrector 34 such that station B, tilt-focuserror compensator 8, and region 342 of the AO wavefront corrector 34,are substantially aligned. Beam 91, resulting from the reflection ofbeam 9 by AO wavefront corrector region 341, is split, by beam splitter39, into a first part which is directed to AO wavefront corrector region342 after successive reflection by reflectors 37, 36 and 35, and asecond part (beam 92) which is directed to WES 34A after successivereflection by reflectors 42 and 43. WES 34A senses the distortion of thebeam, computes the required correction and addresses AO wavefrontCorrector region 341 to drive input distortion to zero or near zero.Corrected beam 91 emerges from AO wavefront corrector region 341,substantially distortion free or at least with reduced distortions. Partof beam 91, i.e. beam 92, is redirected to WES 34A for furthercorrections and so on. This illustrates the functioning of a servo-loopor closed-loop system. Similarly, beam 101, resulting from thereflection of beam 10 by AO wavefront corrector region 342, is split, bybeam splitter 38, into a first part which is directed to AO wavefrontcorrector region 341 after successive reflection by reflectors 35, 36and 37, and a second part (beam 102) which is directed to WES 34B aftersuccessive reflection by reflectors 40 and 41. WES 34B senses thedistortion of the beam, computes the required correction and addressesAO wavefront corrector region 342 to drive input distortion to zero ornear zero. Corrected beam 101 emerges from AO wavefront corrector region342, substantially distortion free or at least with reduced distortion.Part of corrected beam 101, i.e., beam 102 is redirected to WES 34B forfurther corrections and so on.

Each of AO wavefront corrector regions 341 and 342, is driven orcontrolled (i.e., configured, in terms of its pixelated phase map) bythe respective incident optical beam. AO wavefront corrector region 341is controlled by optical beam 9 originating from station A, while AOwavefront corrector region 342 is controlled by optical beam 10originating from station B.

In order to optimize the efficiency of the system, beam splitters 38 and39 are preferably designed to transmit most of the incident light, whilereflecting just enough light then sensed by WES 34B and WES 34A,respectively, so that the WESs can function with a adequatesignal-to-noise ratio.

The purpose of the optical tilt-focus error compensators 7 and 8, is toremove overall tilt and/or focus errors between the pair of compensatedbeams, so that they propagate in exact opposition to each other withinthe system (i.e., they counter-propagate). These compensators 7 and 8may be omitted if the field-of-view and the dynamic range of the AOwavefront corrector 34 provides sufficient correction for theselower-order errors (tilt and focus) without compromising the ability tocompensate for the higher-order wavefront errors on the respective inputbeams. This assumes that the AO wavefront corrector 34 has sufficientdynamic range.

For the purpose of illustration, let us suppose that incident inputbeams 9 and 10 each possesses an arbitrary wavefront error uponincidence onto the respective AO wavefront corrector regions 341 and342. Moreover, let us assume that each beam is independently encodedwith useful (and different) global information, in the form of eitheramplitude or phase modulation. We further assume that the encodedmodulation bandwidth exceeds the adaptive optical closed-loopcompensation bandwidth, so that the desired modulation is preserved,after beam clean-up. The compensation bandwidth must equal or exceed thedistortion effective bandwidth for the system to function. As anexample, atmospheric distortions have a time scale on the order of amillisecond, so the AO compensation bandwidth must be greater than 1KHz. On the other hand, the desired communication bandwidth (or linkdata rate) can be very large (1 to 100 GHz, for example).

The role of the AO wavefront corrector region 341 (342, respectively)and WES 34A (34B, respectively) is to minimize, upon reflection by AOwavefront corrector region 341 (342, respectively), the wavefront errorscarried by the input beam 9 (10, respectively). That is, AO wavefrontcorrector region 341 (342, respectively) will drive the spatial phaseerror φ_(res) of incident input beam 9 (10, respectively) to a smallresidual value dictated by the closed-loop servo gainG(φ_(res)≈φ_(in)/(1+G), where φ_(in) is the input phase error). The gainG usually ranges from about 2 to about 100, with higher values givingbetter system performance. The result of this operation is that a highlyaberrated input beam 9 (10, respectively), will, after reflection by AOwavefront corrector region 341 (342, respectively), emerge as anear-plane wave beam 91 (101, respectively). Note that any global phaseor intensity modulation will remain on the planarized (i.e., thewavefront scrubbed) beam 91 (101, respectively). The planarized beam 91(101, respectively) maintains the globally encoded modulationinformation. This modulated plane wave beam 91 (101, respectively) willthen be reflected by reflectors 37, 36, and 35 (35, 36, and 37,respectively) and finally by the other AO wavefront corrector region,namely AO wavefront corrector region 342 (341, respectively) whichdirects corrected modulated plane wave beam 91 (101, respectively) toits final destination, i.e., station B (station A, respectively). Notethat the cleaned-up beam 91 (101, respectively) does not affect thespatial phase of AO wavefront corrector region 342 (341, respectively)since this region is controlled by incident input beam 10 (9,respectively) originating from station B (A, respectively).

By reciprocity, plane wave beam 91 (101, respectively) will, uponreflection by AO wavefront corrector region 342 (341, respectively),emerge with the same wavefront as beam 10 (9, respectively) had beforeit reflected off AO wavefront corrector region 342 (341, respectively).The encoded input beam 9 (10, respectively) will thus propagate into theprecise reverse direction of beam 10 (9, respectively) and arrive atstation B (A, respectively) as a diffraction-limited beam.Diffraction-limited characterizes a beam with highest focusing ability,and is determined by the ratio, λ/D, where λ is the wavelength and D theaperture. Optical distortions increase this ratio by one to severalorders of magnitude (˜10 to ˜1000, or more) which in turn degradesperformance. Optional amplifiers 190, 191 can be placed in the system(which can also be corrected by the system). The location of theamplifiers also is optional in that a nearly planarized beam will entereach of the amplifiers so that an amplifier with a small FOV willsuffice.

The different types of AO modules suitable for the embodimentspreviously described, include reflective devices such as conventionalpixelated piston-driven membranes (“rubber mirrors”), Liquid CrystalLight Valves (LCLVs) or LC pixelated phase shifters, which can beoptically or electrically driven on a pixel-by-pixel basis, liquidcrystal Spatial Light Modulators (SLMs), deformable MEMS devices, oroptical MEMS-based SLMs. Suitable AO modules may also includetransmission devices such as liquid crystal cells with transparentelectrodes or any combination of the these devices. Conventionalwavefront error sensors may also be used which drive deformable mirrors(e.g., PZT-activated, etc.). Regardless of which devices are used, anincident beam will emerge from each AO module with its wavefrontsplanarized.

The aforedescribed embodiments basically assume that referenceinformation (i.e., a beam that contains the propagation-path phaseerrors) accompanies the data to be transmitted. For example, if thereference signal is interleaved with the desired data or if the desireddata is FM or PM on a carrier signal whose amplitude is controlled bythe disclosed embodiment, then the aforedescribed embodiments shouldwork suitably, but they do have the disadvantage of transmission losswhich arises since at least a percentage of the light is lost by beamsplitters 17 and 18 for the purpose of configuring the AO modules 3,4.

FIG. 6 a is an embodiment similar to the aforementioned embodiments ofFIGS. 3A, 3B and 4. In this embodiment, the beam splitters 17 and 18instead of programming AO modules, need only split off sufficient lightthat an associated detector/processor 17 a, 18 a can differentiatebetween transmitted reference information and the desired data for thepurpose of controlling electro-optic switches 17 b and 18 b. The linesconnecting elements 17 a and 17 b and connecting elements 18 a and 18 bare shown in bold lines to represent the fact that these paths arepreferably provided by electrical connections rather than opticalconnections.

The transmitted data preferably includes frames which each comprise async pulse, followed by reference data, followed by the desired data.The desired data is the information which is to be transmitted fromstation A to B or vice versa. The reference data is data which is knownin advance at both stations A and B and the real purpose in transmittingit is to detect for errors in the communication channel and tocompensate for those errors when the desired data is being transmitted.Compared to the transmitted version of the known-in-advance referencedata, the known-in-advance reference data or signal against which thetransmitted version is compared may be thought of as a idealized versionof the reference data while the transmitted version of the referencedata or signal may be thought of as a distorted version due todistortions occurring in the communication channel.

FIG. 6 b shows a frame of data 56 transmitted (including a sync pulse50, reference data 52 and desired data 54). The desired data 54 cancomprise text, pictures, video, multimedia, or data for any nature whichneeds to be transmitted from point A to B (or vice versa). The nature ofreference data is known in advance by the receiving station. It may besingle valued or it may varied according to a known algorithm.

Aberration in the transmitted signals is induced by regions, forexample, Φ_(A) and Φ_(B), in the optical path which induce suchaberrations.

Detector/processor 17 a, 18 a senses the sync pulses 50 in each frame.The reference data 52 preferably comes next and the electro-opticswitches 17 b, 18 b switch the incoming signal (light) to WES 3 b, 4 bwhere the received reference data is compared with its expected value(s)and the wavefront correctors 3 a, 4 a are adjusted to track out the patherror. Thus, electro-optic switches 17 b, 18 b switch close to 100% ofthe light to the associated WES 3 b, 4 b during receipt of referencedata 52 and allow close to 100% of the light to the travel over thecommunications path when desired data 54 is being transmitted. The patherror is sampled during the time the reference data 52 is received andthen the wavefront correctors 3 a, 4 a are held in the state ofcorrection determined during receipt of the reference data 52 throughoutreceipt of the data of interest 54. Since the path error is typicallycaused by various environmental factors and since these factors changerelatively slowly, making an assumption that the errors detected duringdata period 52 with also be in play (and therefore can still be trackedout) during data period 54 is a reasonable assumption to make.

In a full duplex embodiment, the sync pulses 50 in each half duplexsignal should be in sync with one another so that (i) both electro-opticswitches 17 b, 18 b will be in a position allowing the desired data 54to be exchanged at the same time by stations A and B along path 62 and(ii) both electro-optic switches 17 b, 18 b will be in a position todivert reference data 52 to their respective WES 3 b, 4 b at the sametime. Preferably, syncing occurs as follows: One station (A or B)initiates half-duplex communications and the other station (B or A) thenstarts communicating with the sending station by first timing its frames56 so that its sync pulses 50 to be in sync with sync pulses 50 from theother station within the optical interconnect.

Alternatively, the syncing could occur within the interconnect by addingintentional optical delay in path 62 to superimpose the syncing pulses50 of each sending station one upon the other.

The sync pulse may be a single pulse as graphically shown in FIG. 6 b,or it may be a multi-bit orthogonal code. It may also be combined withreference signal 52, if desired, as opposed to being a separate pulse asshown. In that case, the reference data is effectively being interleavedwith the desired data to be transmitted. The reference data can also beassociated with an encryption algorithm.

FIG. 7 shows another way of transmitted both reference data and desireddata. Instead of distinguishing Φ_(A) and Φ_(B) reference signals fromthe desired link data temporally as done in the embodiment of FIG. 6 a,in this embodiment this signals are distinguished by using differentpolarizations of light for the reference data and for desired link data.This embodiment assumes that each polarization experiences the samedistortion or aberration field, which is the case for most typicalcommunication scenarios. The reference data for station A, for example,is added to the desired data using a polarizing mixer 59 while thereference data for station B is added to the desired data using apolarizing mixer 60. The reference data is orthogonal to the desireddata. The two are separated using a polarization beam splitter 17 d, 18d for each leg. In this way, the reference data for station A isseparated out by splitter 17 d to follow path 17-1 to wavefrontcorrector 3. In a similar vein, the reference data for station B isseparated out by splitter 18 d to follow path 18-1 to wavefrontcorrector 4.

Polarization detectors/correctors 17 c, 18 c may be used in each leg inorder to compensate for the fact that the optical interconnect may wellbe receiving signals from arbitrary directions for which thepolarization of the signals may not be known in advance. Thepolarization detectors/correctors 17 c, 18 c can be used to detect thepolarization of the incoming signals and correct them as needed to workproperly with the disclosed optical interconnect. The lines connectingelements 17 c and 17 e and connecting elements 18 c and 18 e are shownin bold lines to represent the fact that these paths are preferablyprovided by electrical connections rather than optical connections.

In this embodiment, the reference signal preferably is modulated with aunique signal which can be detected. For example, the unique signalcould be a pseudo random code. The detectors 17 c, 18 c would thencontrol the polarization corrector to rotate the polarization in thelegs, for example, such as to maximize the strength of the pseudo randomcode so that the reference signal would indeed be picked off properly bythe downstream polarizing beam splitters 17 d, 18 d.

FIG. 8 depicts another embodiment for separating the reference data fromthe information data to be transmitted. In this embodiment, the two datasets are distinguished by the optical frequency at which they aretransmitted. This embodiment is similar to the polarizationdistinguishing feature of FIG. 7, but instead of using polarization beamsplitters 17 d, 18 d, dichroic beam splitters 17 e, 18 e (or amulti-pixel imaging optical channelizer) are used in each leg.

The disclosed techniques of separating the reference data from theinformational data use temporal, polarization and wavelength techniques.However, there is no reason not to combine these techniques. Forexample, the two wavelengths of the embodiment of FIG. 8 can, inaddition, be time gated differently as described with reference to FIG.6A and/or polarized differently as described with reference to FIG. 7.Using such techniques in combination can make the disclosed system morerobust and/or covert. Also, certain techniques may well work better inthe face of certain types of propagation errors. For example,birefringent and/or dispersive propagation errors may very well limitthe techniques which are used.

The wavelength difference of two sets of data does not need to be widelyspaced. Indeed, standard commercial grade optical communicationequipment should suffice, using, for example, the standard channelseparation for telephony in an optical circuit as being an appropriatechannel spacing between the desired data and the reference data. Thischoice of closely-spaced optical carrier wavelengths is also desirablein the face of dispersion along the propagation path.

Optical tilt-focus error compensators 7 and 8, discussed with referenceto the embodiments of FIGS. 3A, 3B and 4 may also be used with theembodiments of FIGS. 6A, 7 ad 8.

Possible wavefront error sensors include conventional shearinginterferometric sensors, a Shack-Hartmann (local tilt) sensor, or aholographic intensity-to-phase sensor.

Possible global tilt-focus error compensator (used for bore-sighting)include a pair of tilt mirrors (conventional, optical MEMS, etc.), apair of real-time liquid crystal gratings, etc., which are driven by astandard closed-loop quad detector-based servo loop.

The various elements comprised in the interconnection system arepreferably packaged in a compact structure. The distances between thetwo stations and the interconnection system may be large, however.

The system of the present invention acts as an optical interconnect,essentially coupling the two beams that emanate from their respectivetransceivers (stations), while maintaining their encoded information.Each beam leaves the interconnection system (in a spatial sense) in theform of a phase-conjugate replica of the other beam, yet, the temporalencoding on each beam is preserved. Since the pair of AO modules 3 and4, or regions 341 and 342, are locally controlled by input beams 9 and10 respectively, the two beams do not need to be coherent or even havethe same nominal wavelength (the allowed wavelength difference isgoverned by the dispersion and diffraction of the system, and thepropagation path characteristics for a given range). Thus, the systemcan function in the presence of differential Doppler shifted beams,emanating from platforms moving at different speeds, as well as withgeneral beam wander and propagation errors.

The system can also function in guided-wave architectures, providingdynamic coupling of information from one fiber (or waveguide) channel toanother, or to a plurality of channels. All that is required is that agiven channel provide a reference or beacon beam so that the AO modulecan planarize the beam and, at the same time, provide for aphase-conjugate return of the temporally encoded beam back to thereference beam point of origin.

The optical paths shown in the disclosed embodiment are depicted forease of illustration as opposed to necessarily representing the actualpaths that light might take. Also, some optical paths can be shortenedby using electrical paths instead for a portion of an optical path. Forexample, FIG. 5 shows a number of light paths and a number of mirrors toeffect changes of light direction. Various classes of Wavefront ErrorSensors (WES) which involve detection arrays, processors, etc. thatelectronically drive the SLM on a pixel-by-pixel basis can be used toreduce the light paths. Further more, instead of using mirrors, lightcan be fed where desired using a bundle of coherent optical fibers.

While the present disclosure describes various embodiments, theseembodiments are to be understood as illustrative and do not limit theclaim scope. Many variations, modifications, additions and improvementsof the described embodiments are possible. For example, those havingordinary skill in the art will understand that the process parameters,materials, and dimensions are given by way of example only. Theparameters, materials, and dimensions can be varied to achieve thedesired structures as well as modifications, which are within the scopeof the claims. Variations and modifications of the embodiments disclosedwill now suggest themselves to those skilled in the art. The disclosedembodiments best explain the principles of the invention and itspractical application to thereby enable others skilled in the art toutilize the disclosed technology in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the appended claims be construed to include otheralternative embodiments of the invention except insofar as limited bythe prior art.

1. A method of optically interconnecting a first station to a secondstation by coupling a first optical beam and a second optical beam, thefirst optical beam originating from the first station and being directedto the second station, the second optical beam originating from thesecond station and being directed to the first station, the methodcomprising: (a) providing a first adaptive optical module and a secondadaptive optical module; (b) disposing the first adaptive optical modulein a path of the first beam for (i) reflecting and directing the firstbeam to the second adaptive optical module; and (ii) reflecting thesecond beam received from the second adaptive optical module, anddirecting the second beam to the first station; (c) disposing the secondadaptive optical module in a path of the second beam for (i) reflectingand directing the second beam to the first adaptive optical module; and(ii) reflecting the first beam received from the first adaptive opticalmodule, and directing the first beam to the second station; (d)transmitting reference data from the first station towards the secondstation and from the second station towards the first station, thereference data being separable from other transmitted data; (e)comparing the reference data as received with the reference data astransmitted; (f) configuring the first and second adaptive opticalmodules to at least partially compensate for propagation errors inducedin the received reference signal by transmission through at least oneregion of aberration.
 2. The method of claim 1, further including: (a)providing at least one optical tilt-focus error compensator for removingtilt and/or focus errors between the first and second beams so that thebeams counter-propagate; and (b) disposing the at least one opticaltilt-focus error compensator between the first adaptive optical moduleand the second adaptive optical module, such that the at least oneoptical tilt-focus error compensator is in a light path between thefirst adaptive optical module and the second adaptive optical module. 3.The method of claim 2, wherein the at least one optical tilt-focus errorcompensator comprises a first optical tilt-focus error compensator and asecond optical tilt-focus error compensator, the method furtherincluding: (a) disposing the first optical tilt-focus error compensatorbetween the first station and the first adaptive optical module suchthat the first beam passes through the first optical tilt-focus errorcompensator before it reaches the first adaptive optical module; and (b)disposing the second optical tilt-focus error compensator between thesecond station and the second adaptive optical module such that thesecond beam passes through the second optical tilt-focus errorcompensator before it reaches the second adaptive optical module.
 4. Themethod of claim 3 wherein: the first adaptive optical module comprises:a first adaptive optical wavefront corrector; and a first wavefronterror sensor disposed adjacent the first adaptive optical wavefrontcorrector; and the second adaptive optical module comprises: a secondadaptive optical wavefront corrector; and a second wavefront errorsensor disposed adjacent the second adaptive optical wavefrontcorrector.
 5. The method of claim 4 wherein: reflecting and directingthe first beam to the second adaptive optical module is carried out bythe first adaptive optical wavefront corrector; after reflecting acorrected first beam by the first adaptive optical wavefront corrector,a first part of the corrected first beam is directed to the firstwavefront error sensor; the first wavefront error sensor senses adistortion of the first beam, computes a correction, and addresses thefirst adaptive optical wavefront corrector to reduce the distortion ofthe first beam by producing the corrected first beam after reflection ofthe first beam by the first adaptive optical wavefront corrector; aftercorrection of the first beam, and reflection and directing of thecorrected first beam by the first adaptive optical wavefront corrector,a second part of the corrected first beam is directed to the secondadaptive optical wavefront corrector; the second adaptive opticalwavefront corrector reflects and directs the corrected first beam to thesecond station; reflecting and directing the second beam to the firstadaptive optical module is carried out by the second adaptive opticalwavefront corrector; after reflecting of a corrected second beam by thesecond adaptive optical wavefront corrector, a first part of thecorrected second beam is directed to the second wavefront error sensor;the second wavefront error sensor senses a distortion of the secondbeam, computes a correction, and addresses the second adaptive opticalwavefront corrector to reduce the distortion of the second beam byproducing a corrected second beam after reflection of the second beam bythe second adaptive optical wavefront corrector; after correction of thesecond beam, and reflection and directing of the corrected second beamby the second adaptive optical wavefront corrector, a second part of thecorrected second beam is directed to the first adaptive opticalwavefront corrector; and the first adaptive optical wavefront correctorreflects and directs the corrected second beam to the first station. 6.The method of claim 1, wherein the adaptive optical modules compriseLCLVs, liquid crystal SLMs, deformable MEMS devices, optical MEMS-basedSLMs, or liquid crystal cell with transparent electrodes, or anycombination thereof.
 7. A method of creating an optical link between afirst and a second station for the purpose of exchanging informationbetween the two stations, the method comprising: (a) providing a firstoptical beam emanating from the first station, and a second optical beamemanating from the second station; (b) pointing the first optical beamand the second optical beam to a common location; (c) directing eachbeam into a reverse direction of the other so that each station receivesthe beam which emanated from the other station; and (d) correctingpropagation distortions of the first and second optical beams by (i)transmitting reference data from the first station towards the secondstation and from the second station towards the first station, thereference data being physically separable from other transmitted data;(ii) separating the reference data from any other transmitted data andcomparing the reference data as received with the reference data astransmitted; (iii) configuring first and second adaptive optical modulesat said common location to at least partially compensate for propagationerrors induced in the received reference signal by transmission throughone or more regions of aberration.
 8. The method of claim 7 wherein thestep of correcting propagation distortions of the first and secondoptical beams includes planarizing the wavefronts of the first andsecond optical beams, the planarizing the first and second optical beamsbeing carried out by said first and second adaptive optical modules, thefirst and second adaptive optical modules each functioning in aclosed-loop fashion.
 9. The method of claim 8 further includingcompensating for tilt and focus errors of the first and second opticalbeams utilizing at least one optical tilt-focus error compensatortherefor.
 10. The method of claim 9, wherein information is encoded ontothe first optical beam at the first station, information is encoded ontothe second optical beam at the second station, and wherein the firstoptical beam arrives at the second station as a diffraction-limited beamand delivers to the second station the information encoded onto thefirst optical beam at the first station, and the second optical beamarrives at the first station as a diffraction-limited beam and deliversto the first station the information encoded onto the second opticalbeam at the second station.
 11. An interconnect for opticallyinterconnecting a first station and a second station, the interconnectcomprising: a first adaptive optical module positioned in the line ofsight of the first station; a second adaptive optical module positionedin the line of sight of the second station and in the line of sight ofthe first adaptive optical module; and comparators for comparing atransmitted version of a reference signal with a known in advanceversion of the reference signal and for adjusting the first and secondadaptive optical modules to account for propagation errors occurringbetween the first and second stations.
 12. The interconnect of claim 11,wherein the propagation errors are corrected by the first and secondadaptive optical modules, and wherein the first and second adaptiveoptical modules function in a closed-loop fashion.
 13. The interconnectof claim 11, wherein: the first adaptive optical module (i) directs tothe second adaptive optical module, a first optical beam received fromthe first station, and (ii) directs to the first station, a secondoptical beam received from the second adaptive optical module andoriginating from the second station; and the second adaptive opticalmodule (i) directs to the first adaptive optical module, the secondoptical beam received from the second station, and (ii) directs to thesecond station, the first optical beam received from the first adaptiveoptical module and originating from the first station.
 14. Theinterconnect of claim 13 further comprising at least one opticaltilt-focus error compensator for removing tilt and focus errors from atleast one of the first and second optical beams.
 15. The interconnect ofclaim 14 wherein the at least one optical tilt-focus error compensatorcomprises a first optical tilt-focus error compensator and a secondoptical tilt-focus error compensator, the first optical tilt-focus errorcompensator being disposed between the first station and the firstadaptive optical module such that the first optical beam passes throughthe first optical tilt-focus error compensator before reaching the firstadaptive optical module; and the second optical tilt-focus errorcompensator being disposed between the second station and the secondadaptive optical module such that the second optical beam passes throughthe second optical tilt-focus error compensator before reaching thesecond adaptive optical module.
 16. The interconnect of claim 14 whereinthe at least one optical tilt-focus error compensator is disposedbetween the first adaptive optical module and the second adaptiveoptical module, such that the at least one optical tilt-focus errorcompensator is in a light path between the first and second adaptiveoptical modules.
 17. The interconnect of claim 16 wherein: the firstadaptive optical module comprises: a first adaptive optical wavefrontcorrector; and a first wavefront error sensor disposed adjacent thefirst adaptive optical wavefront corrector; and the second adaptiveoptical module comprises: a second adaptive optical wavefront corrector;and a second wavefront error sensor disposed adjacent the secondadaptive optical wavefront corrector.
 18. The interconnect of claim 17further comprising: a first beam splitter for splitting the firstoptical beam, the first beam splitter being disposed in a light pathbetween the first and second adaptive optical modules; and a second beamsplitter for splitting the second optical beam, the second beam splitterbeing disposed in a light path between the first and second adaptiveoptical modules.
 19. The interconnect of claim 18 wherein: the firstadaptive optical wavefront corrector directs the first optical beam tothe second adaptive optical module by reflecting a corrected first beam;after reflecting of the corrected first beam by the first adaptiveoptical wavefront corrector, a first part of the corrected first beam isredirected by the first beam splitter to the first wavefront errorsensor; the first wavefront error sensor senses a distortion of thefirst beam, computes a correction, and addresses the first adaptiveoptical wavefront corrector to reduce the distortion of the first beamby producing the corrected first beam after reflection of the first beamby the first adaptive optical wavefront corrector; after correction ofthe first beam, and reflection and directing of the corrected first beamby the first adaptive optical wavefront corrector, a second part of thecorrected first beam is transmitted by the first beam splitter to thesecond adaptive optical wavefront corrector; the second adaptive opticalwavefront corrector reflects and directs the corrected first beam to thesecond station; the second adaptive optical wavefront corrector directsthe second optical beam to the first adaptive optical module byreflecting a corrected second beam; after reflecting of a correctedsecond beam by the second adaptive optical wavefront corrector, a firstpart of the corrected second beam is redirected by the second beamsplitter to the second wavefront error sensor; the second wavefronterror sensor senses a distortion of the second beam, computes acorrection, and addresses the second adaptive optical wavefrontcorrector to reduce the distortion of the second beam by producing acorrected second beam after reflection of the second beam by the secondadaptive optical wavefront corrector; after correction of the secondbeam, and reflection and directing of the corrected second beam by thesecond adaptive optical wavefront corrector, a second part of thecorrected second beam is transmitted by the second beam splitter to thefirst adaptive optical wavefront corrector; and the first adaptiveoptical wavefront corrector reflects and directs the corrected secondbeam to the first station.
 20. The interconnect of claim 18 wherein: thefirst adaptive optical wavefront corrector directs the first opticalbeam to the second adaptive optical module by transmitting a correctedfirst beam; after transmission of the corrected first beam by the firstadaptive optical wavefront corrector, a first part of the correctedfirst beam is redirected by the first beam splitter to the firstwavefront error sensor; the first wavefront error sensor senses adistortion of the first beam, computes a correction, and addresses thefirst adaptive optical wavefront corrector to reduce the distortion ofthe first beam by producing the corrected first beam after transmissionof the first beam by the first adaptive optical wavefront corrector;after correction of the first beam, and transmission of the correctedfirst beam by the first adaptive optical wavefront corrector, a secondpart of the corrected first beam is transmitted by the first beamsplitter to the second adaptive optical wavefront corrector; the secondadaptive optical wavefront corrector transmits the corrected first beamto the second station; the second adaptive optical wavefront correctordirects the second optical beam to the first adaptive optical module bytransmitting a corrected second beam; after transmitting of a correctedsecond beam by the second adaptive optical wavefront corrector, a firstpart of the corrected second beam is redirected by the second beamsplitter to the second wavefront error sensor; the second wavefronterror sensor senses a distortion of the second beam, computes acorrection, and addresses the second adaptive optical wavefrontcorrector to reduce the distortion of the second beam by producing acorrected second beam after transmission of the second beam by thesecond adaptive optical wavefront corrector; after correction of thesecond beam, and transmission and directing of the corrected second beamby the second adaptive optical wavefront corrector, a second part of thecorrected second beam is transmitted by the second beam splitter to thefirst adaptive optical wavefront corrector; and the first adaptiveoptical wavefront corrector transmits the corrected second beam to thefirst station.